Through-tubing carbon-oxygen (C/O) logging is a popular means for reservoir monitoring since a slim logging-inside-casing tool was introduced. The tools are available in 2.5 inch and 1.6875 inch outer diameters. Both the tools log inside of casing to quantitatively determine fluid saturation within the formation. In the absence of any other external information, the C/O technique has limitations when the survey is carried out inside the tubing. However, at times this becomes necessary when the reservoir to be evaluated is covered with both tubing and casing.

There has been limited use of the slim logging-inside-casing tool for completions with both tubing and casing. This application has augmented the C/O methods of saturation estimation for measurements made inside the tubing.

The present work documents the results of a pulse neutron logging operation carried out for estimation of the remaining oil saturation of a reservoir covered with tubing and casing. Both the inelastic capture and the sigma survey were recorded in this well. Careful planning was carried out for achieving an oil saturation precision of around 7 saturation units. The interpretation of the lithology from both the open and the cased hole logs is discussed. A new method was used for estimating the permeability from the cased hole spectroscopy logs. The saturation profile estimated from the C/O log is integrated with the estimated permeability results.

The saturation results are in agreement with field observations and also demonstrate that bypassed reserves can be estimated on the basis of pulse neutron logging carried out inside tubing.